Inkjet desktop and laser printers are widely used to individually print paper materials. These printers do, however, suffer from the drawback that the inks may fade with time and may offer only limited resistance to water, dirt, chemicals, UV radiation, and mechanical impact. As a result, such printed materials may be unsuitable for long-term applications either indoors or outdoors. Moreover, if exposed to air for a fairly long period of time, paper labels can become discolored, with the paper typically turning yellow.
To make materials, such as labels that have been printed by means of an inkjet desktop or laser printer, suitable for long-term applications, methods are being sought that improve the resistance of the label and its imprint with respect to the aforementioned possible influences.
To enhance a label's resistance to water, special-purpose inkjet coatings can be applied to paper. Such coatings may also simultaneously improve the quality of printing. Such an approach, however may not achieve a degree of water resistance that would permit use outdoors. Moreover, light resistance is typically not enhanced by the special-purpose coating. In consequence, an indoor or outdoor application which lasts several months usually causes the inks to fade to such an extent that it may no longer be possible to read or to discern the indicia printed thereon.
The resistance of materials can also be improved by lamination using a film. For example, DE 1 296 363 describes a process in which a sheet of paper, while being heated, is bonded to a flexible, tough and transparent plastic film by means of an adhesive coating on that side of the plastic film which faces towards the sheet of paper. This kind of plastic film is resistant to solvents, such as acetone, carbon tetrachloride and heptane, and is resistant to degradation when exposed to water. However, the process can suffer from the disadvantage that the lamination technique can be relatively complicated and the plastic coating typically first has to be activated at 50° C. to 85° C. before bonding occurs.
In addition, it is known that pigmented inks should be used in the case of industrial printers in order to enhance the resistance of the inks to light. Nonetheless, the use of such pigmented inks, apart from the color black, has so far not been feasible in inkjet desktop printers.
Furthermore, DE 40 03 129 describes a styrene-based film that contains a benzophenone type or benzotriazole type ultraviolet (UV) absorber and/or a sterically hindered amine type light stabilizer. The products are free of fish-eye gel, do not discolor in conditions of humid heat, can be readily printed and are resistant to weathering. Nevertheless, there is the disadvantage that styrene-based films typically can be printed only by using a special-purpose printing ink for polystyrene, and not by using commercially available inkjet desktop or laser printers.
It is, moreover, known in principle that a label's resistance can be improved by lamination using a transparent film. Nevertheless, this kind of lamination is typically relatively complicated because the label and the film are present as separate sheets or rolls and the film is typically tailored to the size of the label in an additional procedural stage. Since, moreover, lamination may take place on the basis of a visual estimation, it is frequently the case that the label is bonded lopsidedly with the film, which detracts from the appearance of the resultant laminated label. Typically, it is not possible to reposition the film because once bonded with the surface of a paper label, the adhesive may stick tightly and inhibit the film from being peeled off again. Additionally, large-format labels, in particular, suffer from the problem that during bonding, air bubbles or pockets may form between the label and the laminating film; such air pockets likewise can cause the label to look unattractive.
To permit a label to be laminated with a film in as positionally accurate a manner as possible, the Nichiban company developed a label sheet in which the label and the transparent laminating film are integrated within a single sheet. This label sheet comprises the following layers: laminating film, pressure-sensitive adhesive layer, release layer, paper layer. The label sheet further comprises punch lines, by means of which sub-regions the size of a label are formed. The sub-regions that are formed in the paper layer, including the release layer, are smaller than the sub-regions formed in the laminating film, including the pressure-sensitive adhesive layer, the latter protruding over the former at the edges. To laminate a label, the sub-region corresponding to the label is removed from the paper layer and, having been turned upside down, re-affixed into the resultant window onto the pressure-sensitive adhesive layer, thereby causing the printable layer to be bonded to the pressure-sensitive adhesive layer. Afterwards, the sub-region of the laminating film, together with the stuck-on label, is removed from the label sheet. The laminating film's protruding edges, which project over the stuck-on label, cause the composite to be in turn affixed to any kind of object.
Nichiban's above-described label sheet, while generally useful, does suffer from the drawback that typically only relatively small labels can be bonded effectively in this way. Larger labels are faced with the problem that, during lamination, air pockets may be trapped between the label and the laminating film, which can detract from the appearance of the resultant laminated label. Furthermore, it is typically not possible to re-adjust the position of a paper label once it has been stuck on.
U.S. Patent Application Publication No. 2002/0011306 describes adhesive labels which provide areas of non-adhesive material within the adhesive layer to allow the removal of entrapped air pockets. For the same reason, the adhesive layer may be provided with a Sheffield-roughness of at least 10 units or at least 70 units or at least 150 units.
German Patent Application Publication DE 197 24 648 A1 describes a self-adhesive area-measured product comprising a substrate, a non-stick coating on the upper side of the substrate, and an adhesive coating on the underside of the substrate, and being characterized in that the adhesive coating is present in the form of a pattern and that the upper side of the substrate having the non-stick coating has a roughness of at least 1 μm (measured according to ISO 8791-4).
Hence, there is a need for a label sheet that can be used to laminate labels with a film in a simple, effective, and accurate manner, such that the labels are made resistant, for example to light, UV radiation, ozone, dirt, water, chemicals and mechanical impact, and are thus suited to long-term application both indoors and outdoors. There is additionally a need for labels that are relatively free of entrapped air pockets between the laminating film and the printable layer, particularly whenever large-format A4 or A3 labels are used. There is also a need to be able to reposition the laminating film if so desired. The present invention addresses one or more of these needs.